Aerogels, piezoelectric devices, and uses therefor

ABSTRACT

In its first aspect the invention provides an electromechanical acoustic transducer ( 5 ) which is resistively terminated at the rear with a closely-coupled sound absorber ( 7 ) made from an aerogel with good acoustic absorption at low frequencies. The invention also provides an aerogel suitable for this use and which is a conglomerate of small particles packed so that there are spaces left therebetween to allow the passage of sound. Moreover, the invention provides an aerogel suitable for this use and comprised of multiple layers ( 17   a  etc.) of materials of graded properties, or comprised of material of continuously varying properties throughout the thickness of the absorbing structure, in the principal direction of sound from the source.

This invention relates to aerogels, piezoelectric devices, and usestherefor. More particularly, the invention relates to novelconstructions made from aerogels, to the use of aerogels as resistiveterminations in acoustic transducers, to the use of aerogels astranslators in electric motors and transducers, to the use of aerogelsas pistons in acoustic transducers, to the use of aerogels as transducerelements in visual display units, and to novel constructions ofpiezoelectric devices, some with integral positioning and controlmechanisms, some for use as print-head devices, and some for use aselectromechanical drivers.

AEROGELS AND THEIR USES

A conventional gel is a mixture of a rubbery solid substance, such asgelatine, which forms a continuous phase within which is dispersed, andtrapped, a liquid, such as water. The result is a jelly (or jello). Anaerogel might be described as a solid/gas analogue of a gel; it is asubstantially solid porous (vesicular) material in which some suitablesolid substance forms a continuous phase holding within itself, in poresrather like open-cell “bubbles” (vesicles), a gas (typically air).Pumice, which is a fine solid foam of air in solidified lava(technically it is a “vesicular glass”) usually having a density lessthan one (so it floats in water), is a natural material similar in someways to an aerogel, though the pores in the latter tend to be of randomform and disposition rather then “bubble”-like.

Many synthetic, man-made, aerogels are known. Most are based on silica(silicon dioxide), but there are many made from other materials, such asmetal oxides, and a variety of plastics (like polyurethanes) and naturaland synthetic rubbers. They are commonly manufactured by making a gel ofthe solid and some suitable liquid (such as water), and then removingthe liquid in such a way that the surface tension forces of the liquiddo not collapse the gel structure, as usually happens if a gel isallowed to dry out without special precautions. Typically, this involvesdisplacing the gel's liquid component with alcohol, then displacing thealcohol in turn with liquid CO₂. The gel is then subjected to atemperature and pressure high enough to take the CO₂ into itssuper-critical state (when it is effectively neither liquid nor gas), atwhich point is may be vented from the gel leaving behind the solidphase. The absence of surface-tension forces in the super-critical fluidensures that the solid phase is not collapsed during the venting stage;thus, this leaves the solid phase of the gel largely intact andinterconnected, and in a highly porous state—an aerogel.

In general, aerogels are highly porous. They have pore sizes typicallyin the range 5-50 nanometers, and porosities in the range 50-99.9%.

Depending on what they're made from, and exactly how they're made, soaerogels can have a wide range of physical properties. They can be light(like that silica variety available as SP-50 from Matsushita, whichweighs somewhat less than 185 g/l while the silica from which it isformed is itself much heavier, at about 2.2 kg/l) or extremely light(like the silica material prepared by Larry Husbresh, of the LawerenceLivermore National Laboratory, USA, in the late 1980s, which weighsabout 3 g/l, or only about three times the density of air at NTP,despite the silica from which it is made weighing as much as 2.2 kg/l).They can have large or small cells (vesicles, or pores)—ranging from afew hundred nanometers down to as little as a few nanometers in averagediameter. They can be magnetic (or, at least, easily magnetised oraffected by a magnet), such as the silica aerogel/transition metalcomposites made by the Microstructured Materials group at the BerkleyLab (USA), or they can be substantially non-magnetic, as is the casewith most others. They can be opaque (to visible light) or eventransparent (such as Matsushita's SP-50).

Many aerogels are described in some detail, with their methods ofpreparation, in Aerogels 5, Proceedings of the Fifth InternationalSymposium on Aerogels, (ISA-5), Montpellier, France, 8-10 September1997: Editors J Phalippou, R Vacher (North Holland Elsevier, 1998).Specific aerials there described include those mentioned above(Matsushita at pages 369, Hoeschst at page 24), together with (on page36) an ICI polyurethane/polyisocyanate aerogel in both monolithic andparticle form with a density of 80-100 kg/m³ and a pore size of 11-18nanometers—

AEROGELS AS SOUND ABSORBERS

One aspect of this invention relates to the use of aerogels assubstantially resistive terminations in acoustic transducers.

An acoustic transducer—as typified by either a loudspeaker or amicrophone—is a device that converts electrical energy into audiblesound energy (or, in the case of a microphone, vice versa). They aregenerally electromechanical devices; in a loudspeaker, for example, adiaphragm is driven by a “translator”—a moving coil or a magnet itselfdriven by an applied magnetic field—in a manner dependent on theelectrical signal to be transduced, and the result is that the diaphragmcauses air pressure changes that reproduce the sound represented by theelectrical signal.

Loudspeakers—transducers for acoustic use like this—are most commonly ofthe light-weight moving-coil design, and can be made to operate up toand beyond the limit of human hearing (20,000 Hz or more). However, atthe low end of the range, e.g. below 100 Hz, there arise acousticproblems unrelated to the electromechanical nature of the transducer,which problems are due to the dipole nature of any simpleelectroacoustic transducer. Thus, because the sound radiation—the airpressure waves—from the rear of the transducer is nominally of equalamplitude but in antiphase to that from the front of the transducer,then—and this is especially a problem at low frequencies, where thewavelengths involved are on a par with, or larger than, the dimensionsof the transducer—the essentially omnidirectional rear radiationdestructively interferes with the front radiation, causing a significantloss of useful output.

One standard solution to this problem is to mount the transducer in thefront wall of a closed box, thus stopping the rear radiation fromescaping from the box, and thus preventing the destructive interference.Unfortunately, conveniently-sized (generally, small) boxes have a volumethat is only a relatively small multiple of the volume displaced bymotion of the transducer, and thus with such a closed box the airwithin, having nowhere to escape, becomes significantly compressed anddecompressed as the transducer moves back and forth a significantdistance (as it must at low frequencies if it is to produce any realacoustic power). The net effect is that the closed volume of trapped airacts as a capacitative load on the transducer, and reduces the stroke athigh displacements for a given input power to the transducer. This inturn produces non-linearity and distortion in the output sound.

An alternative way of looking at the problem is to consider thedimensions of the box. Where the internal dimensions of the box aresmall compared to a wavelength of sound from the rear of the transducer,the box, being rigid, reflects the sound back to the transducer. Thisreflected sound is largely in-phase with the rear-radiated sound, andthis therefore loads the transducer so as greatly to reduce itsamplitude of motion (compared to that without the box), thus reducingthe useful sound output from the front of the transducer.

Additionally, closed boxes have relatively strong resonances, and mucheffort has to be expended to reduce these resonances in practicalloudspeakers.

A variation of the closed box approach is to fit a bass-reflex port—thatis, essentially an acoustic tuned circuit that aims to reverse the phaseof the rear-radiation over a narrow frequency band before it reaches thefront of the transducer. This solution is used in some practicalloudspeakers, but has drawbacks due at least in part to itsresonance-related basis, and is known to cause severe phase distortion.An alternative answer to the rear-radiation problem is the so-calledinfinite-baffle—essentially simply a large flat panel (through which ismounted the transducer) the dimensions of which are comparable to orlarger than a wavelength of the lowest frequency of sound to bereproduced. Here, the rear radiation has such a long path to traverse(around the edge of the panel) before it can interfere with the frontradiation that it can no longer significantly interfere destructively.However, the flat panel answer is impractical at low frequencies as thewavelength of sound in air at 20 Hz is close to 50 feet (about 15meters), and obviously a baffle of this size is awkward and unrealisticfor most purposes.

In its first aspect, the invention proposes a possible solution to thisproblem of electromechanical acoustic transducers suffering from rearradiation destructively cancelling the front radiation, particularly atlow frequencies where the path length from front to rear is comparableto or less than a wavelength of sound in air. More specifically, theinvention suggests that the transducer be resistively terminated at therear with a closely-coupled sound absorber (in a manner analogous to thetermination of an electrical transmission line with a matched electricalresistance to avoid reflections, terminating the rear of the transducerwith an approximately matched acoustic resistance can be arranged tocause a high percentage of its rear radiation to be absorbed, and thusunavailable to interfere destructively with the front radiation). Morespecifically still, the invention suggests that the material used as theacoustic resistance be an aerogel of a form known to have good acousticabsorption at low frequencies.

In its first aspect, therefore, the invention provides anelectromechanical acoustic transducer which is resistively terminated atthe rear with a closely-coupled sound absorber made from an aerogel withgood acoustic absorption at low frequencies.

The invention provides in this aspect an electromechanical acoustictransducer. This transducer may be of any required form, and for anypurpose; a typical transducer for use as a hi-fi loudspeaker is of themoving-coil type such as that sold by Audax of France under the nameATO80MO.

The transducer of the invention is resistively terminated at the rearwith a closely-coupled sound absorber (by “closely-coupled” is meantthat substantially all of the sound emitted by the [rear of] thetransducer is caused to be directed towards and into the soundabsorber). In general resistive termination is accomplished in practicemerely by placing behind the transducer, and in the path of any soundradiation emanating from the transducer's rear, a pad, buffer, cushion,wad or block of the requisite sound absorbing aerogel material.Preferably, however, the transducer is placed at and across the front ofa suitably-shaped box open at the rear, and the rear is then closed withthe block of aerogel sound absorber.

The aerogel naturally is one with good acoustic absorption at lowfrequencies. Aerogels made from silica, polyurethanes and rubbers haveparticularly good intrinsic sound absorbing properties. The pore size inthe aerogel can be made as small as several nanometers, much smallerthan the mean free path of oxygen and nitrogen molecules in the air atNTP, which ensures that every molecule in the air driven back and forthby the transducer will, in passing through a plug of aerogel ofcentimeter dimensions, make a very large number of collisions with thematerial of the aerogel, and thus exchange its energy with the material.Internal surface areas in the areogel can be as high as 1,000 m²/g ofmaterial, this providing potentially high sound absorption. Byappropriate choice of the material from which the areogel is made, andthe mean pore size within the material, it is possible to produceacoustic resistances of a wide range of values with very high soundabsorption in small quantities of material, thus making the materialideal for use in the invention to reduce of nearly eliminate the rearradiation, without significant back pressure effects on the transducertranslator, thus producing little or no impairment to the linearity ofthe transducer. It is only necessary to absorb half the acoustic outputof the rear of a transducer to reduce the effects of destructiveinterference at the front of the transducer to a mere 3 dB drop inresponse: in practice significantly more than half the output can beabsorption in the way described, thus eliminating the need for a closedbox or large baffle and allowing a compact wide range acoustictransducer of low weight and wide flat frequency response.

Typical instances of usable aerogel materials are Matsushita's SP-30 (in2-3 mm bead form), SP-15 (also in 2-3 mm bead form) and Hoechst's HIL2(in particle form).

The use of an aerogel sound absorber as described above may be enhancedby the addition of a coupling and filling gas or gas mixture trapped byan outer gas-impermeable but acoustically-transparent membrane, the gasor gases being chosen so as to optimize the matching of the acousticresistance of the absorber to the radiation resistance of thetransducer, or instead, or as well as, to optimize the absorption of thesound from the rear of the transducer.

For example, neon has a viscosity about twice that of air of NTP, whilehydrogen and helium have lower viscosities than air. Sulphurhexafluoride has a sound speed of only 133 m/s, almost 3 times lowerthan air at NTP, whilst Freon 113 has an even lower sound speed of 124m/s. If, then, the space between the rear of the transducer and theoutside air, including the space occupied by the aerogel sound absorber,is partitioned off with a gas-impermeable membrane that is alsoacoustically transparent, and is filled with a suitable mixture of gasesother than air, additional beneficial impedance matching and absorptionenhancements may be obtained, allowing more efficient operation of thetransducer, and more absorption in the same volume or a smaller volumeof absorber to achieve the same level of performance.

If, for instance, helium is used, then firstly the transducer dissipatesless sound energy into the helium because the transducer/gas impedancemismatch is increased, and secondly, because the helium is less viscousthan air, and so flows more readily than air through the pores of theaerogel and other absorptive materials, it increases the soundabsorption in those absorbers. So, for two quite separate reasons thetotal amount of sound energy emanating from the rear of the transducerand absorber combined is reduced when using helium as the gas.

PREFERRED AND NOVEL AEROGEL STRUCTURES

The invention suggests that the transducer be resistively terminated atthe rear with a closely-coupled sound absorber and that the materialused as the acoustic resistance be an aerogel of a form known to havegood acoustic absorption at low frequencies. However, the fabrication ofa practical acoustic matched terminator (or absorber) as described aboveis not easy, for the two prime physical characteristics important—theterminator's impedance and its absorption properties—tend to be theresult of mutually-exclusive structural attributes; the properties ofany one particular type of aerogel material are thus unlikely to haveboth the desired impedance and the desired absorption properties. Thematter is further explained as follows.

Impedance may be described as the innate ability of the material toallow the sound waves to pass therethrough, and if those waves are topass “cleanly” from the air to the material without any hindrance, andas though the material was just more air, then the impedance of thematerial has to match the impedance of the air. However, if theimpedances are not matched then the material behaves as though it weredifferent from air, and there is formed in the medium through which thesound travels a discontinuity at the interface between the two media(air and aerogel terminator), and at the discontinuity it is likely thatsome of the sound will be reflected back (to the transducer) rather thantraveling on into the terminator (and being absorbed). Thus, the effectof the material having an acoustic impedance significantly differentfrom that of air is to produce a significant reflection of incidentacoustic waves (i.e. the reflection coefficient R>>0.0), and inconsequence a correspondingly significant amount of sound energyradiated by the rear of the acoustic transducer towards such a materialwill be reflected back towards the transducer and interfere with itsoperation in an undesirable manner. The reflection coefficient ofaerogel is also a strong function of the macro physical form (ratherthan just the micro material composition) of the material: in particularit is found that monolithic aerogel even of very low density is stillhighly acoustically reflective in the 20 Hz-20 KHz band; the same isalso true of aerogel ground into very fine powder form.

The problem, then, is to construct an aerogel absorber which has a lowreflection coefficient—which is “impedance matched” to the air,providing only a trivial discontinuity effect—while at the same timehaving a high absorption capability. Unfortunately, in practice it isfound that materials that have very low acoustic reflection coefficientsR tend to have low absorption coefficients A too. In consequence, whilstthey produce little reflected energy to interfere with an adjacenttransducer, they do little to absorb the rear acoustic energy either.The converse is also found to be true in practice, i.e. that materialswith high absorption coefficients A also have high reflectioncoefficients R. Thus, such latter materials absorb a good fraction ofthe acoustic energy that gets into them but in turn reflect a lot of theincident acoustic energy at their surfaces.

AEROGELS FROM PARTICLES WITH GAPS

A partial solution to this problem of optimizing the performance of theabsorber is constructing the aerogel in the form of small particles withsizes in the several millimeter region, but arranging that the shapes ofthe particles are such that when the particles are packed there arestill adequate spaces left between the particles to allow the passage ofsound.

In another aspect, therefore, this invention provides an aerogel,suitable for use as an absorber of sound energy, which is constructed asa conglomerate of small particles with sizes in the several millimeterregion with the packing of these particles being such that there arespaces left therebetween to allow the passage of sound.

The aerogel of this aspect of the invention is one which is formed fromsmall but significant particles of aerogel material rather than fromeither a monolithic—all one piece—material or a finely-powderedmaterial. The particles, which may cover a range of sizes and shapes,are preferably in the region of 0.5-20 mm across.

This aerogel is one which is constructed as a conglomerate of smallparticles which pack in such a way that there are spaces lefttherebetween to allow the passage of sound. Particle shapes which closepack—eg, cubes—are ineffective, as when so packed they behave like amonolithic aerogel, and have high reflectivity. Particle shapes which donot close pack include spheres and cylinders, and irregular particlesizes as are produced by crushing larger particles or monoliths. If, inaddition to possessing a shape which does not close-pack, on the scaleof a few millimeters, the particles are also rough-surfaces (on a muchsmaller scale), then they absorb sound even more effectively, and arepreferred. Such particles when formed into a layer produce interstitialgaps in the size range between zero (where they happen to touch on flator convex surfaces) and several millimeters.

AEROGELS THAT ARE GRADED

Another way of solving the impedance/absorbance dilemma is to layer theaerogel. If the aerogel absorber is fabricated such that, in theprinciple direction in which the sound energy to be absorbed travelsthrough the absorber, its relevant properties are graded—eithercontinuously or stepwise—from one side (say, the input side) to theother (the output side), then on the input side (where the sound energyfrom the acoustic transducer is incident) the material may be given thelowest possible reflection coefficient R, and on the output side (theside of the absorber furthest from the acoustic transducer) the materialmay be given the greatest possible absorption coefficient A consistentwith an acceptably-low reflection coefficient. The result of this isthat there are in effect multiple layers of material ofgradually-increasing reflection coefficient in the direction from theinput to output side of the absorber, which provides an overall lowreflectivity which is a significant improvement on a uniform material.This is because very little sound is reflected at the input sideinterface because there the reflection coefficient is chosen to belowest (though, because of the limitations of practical materials, ingeneral this material will have a low absorption coefficient too) so thebulk of the incident sound will be transmitted on to the followinglayers of the absorber. Then, as the sound energy gets a little furtherinto the absorber, it meets material having a slightly higher reflectioncoefficient but which also absorbs more strongly. Here a certain amountof sound energy is reflected back towards the input surface but thiswill be partially absorbed by the intervening material and partiallyreflected back towards the output side. As the sound progresses furtherinto the absorber this process of partial reflection and partialabsorption continues with successively less being transmitted through tothe following layers of material, and more and more of the energyreflected back towards the input being absorbed and/or reflected backtowards the output surface. Thus, it will be seen that there can be madea suitably-structured anisotropic and inhomogeneous absorber structurethat is much superior to any isotropic and homogeneous absorber made ofthe same material.

In yet another aspect, therefore, this invention provides an aerogelsound absorber as described above comprised of multiple layers ofmaterials of graded properties, or comprised of material of continuouslyvarying properties throughout the thickness of the absorbing structure,in the principal direction of sound from the source.

More particularly, in this aspect the invention provides an aerogel,suitable for use as an absorber of sound energy, which is formed as acontinuously or stepwise/layered graded structure from one side to theother, and wherein on the input side (where the sound energy from theacoustic transducer is incident) the material is given the lowestpossible reflection coefficient R (and the maximum possible concomitantabsorption coefficient A), and on the output side (the side of theabsorber furthest from the acoustic transducer) the material is giventhe greatest possible absorption coefficient A consistent with anacceptably-low reflection coefficient.

The graded aerosol structure is made from a set of n aerogel (or other)materials a₀, a₁, . . . a_(n), with steadily increasing reflectioncoefficients R₀, R₁, . . . R_(n) and steadily increasing absorptioncoefficients A₀, A₁, . . . A_(n) (these materials are prepared from theset of all available low-reflection coefficient absorber materials; ifany material to hand has the same or lower reflection coefficient R thanany of the R_(i), i=0 to n, but a higher absorption coefficient, it issubstituted for that R_(i)). Examples of materials a₀, a₁, a₂, . . .a_(n) are the bead form of Matsushita SP-15, SP-30 and SP-50respectively, and Hoechst HIL2.

A functional stepwise multilayer aerogel absorber may be made byoverlaying successive layers of material a₀, a₁, a₂, . . . a_(n), eachwith thickness t₀, t₁, t₂, . . . t_(n), onto a supportive substrate thatis substantially acoustically transparent over the frequency range ofinterest—eg, an open metal or plastic mesh or foam with hole or poresizes smaller than the particle sizes of material forming the absorber.The resulting layered absorber has thickness t₀+t₁+t₂+. . . +t_(n) inthe nominal direction of sound incidence. The individual thicknesses t₀,t₁, t₂, . . . t_(n), be determined in a number of ways. For instance,trial and error experiments, starting from the situation of t₀=t₁=t₂=. .. =t_(n), may be used to improve the absorption. Calculations, using thelayered sound-absorption-and-reflection theory in J F Allard's“Propagation of sound in porous media”, may also be used to optimize thelayer thickness. However, the theory relating to aerogels is still poor,and experiments are essential.

A functional continuous “multilayer” absorber may be constructedsimilarly, except that the proportions of each material overlaid on thesubstrate are continuously varied as the layer builds up. Initially,100% of the material is a₀, but an increasing proportion of a₁ is laiddown such that when the total layer thickness is t₀, 100% of a₁ is beingdeposited. At that point the proportion of a₂ being deposited is slowlyincreased such that as the deposited layer thickness reaches to₀+t₁,100% of a₂ is being deposited. This process is continued, material bymaterial, until finally as the deposited layer thickness reachest₀+t₁+t₂+. . . +t_(n−1), 100% of material a_(n) is being deposited. Inthis case the total layer thickness of the multilayer absorber ist₀+t₁+t₂+. . . +t_(n−1).

One application of these techniques uses as materials a₀, a₁, a₂, . . .a_(n) Matsushita aerogels SP-15, SP-30 and SP-50 in bead form (the beadsare cylinders approximately 2 mm long and 1 mm diameter). First, thereis constructed an acoustically-transparent supportive substrate—in theform, say, of a 300 mm by 300 mm by 40 mm basket of fine stainless steelwoven wire mesh with holes ≅0.9 mm and wire size ≅0.1 mm diameter. A 13mm thick layer of SP-15 is uniformly deposited over the mesh, and overthat is deposited a 14 mm layer of SP-30, followed by a 13 mm layer ofSP-50. The “basket” is then closed with a further piece of the same wiremesh (≅300 mm by 300 mm). In operation, the sound to be absorbed isallowed to impinge on the face whereon is deposited the SP-15. Anabsorption of approximately 8 dB down to a frequency of around 30 Hz isachieved, with a reflection coefficient smaller than 0.2. At higheraudio frequencies, the reflection coefficient is smaller than 0.2, andthe absorption substantially greater than 8 dB.

In the aerogel absorber structures of the invention wherein aparticulate aerogel material is one of the components it is preferableto stabilize the structure by slightly bonding the particles together,conveniently with an adhesive. Suitable such binding adhesives areavailable from Hoechst, of Germany. The choice and quantity of theadhesive binder material can significantly (and often detrimentally)affect the structure's acoustic properties; most preferably, then, verysmall amounts are used.

Although, in the graded aerogel sound absorber of the invention, eachand all of the layers in the structure can be aerogel material,nevertheless sound absorber performance may be further enhanced by usingmaterials other than aerogels to form one or more additional layers.Such additional, non-aerogel layers are preferably those layers furtheraway from the sound source, so that the non-aerogel material's higherreflection coefficient is effectively shielded from the sound source bythe intervening layers of low reflection coefficient aerogel. Thenon-aerogel layers may be made from any appropriate material. Particularinstances of such non-aerogel materials suitable for use as one of thehigher absorption, higher reflection coefficient layers in such amultilayer sound absorber are ASTRON ASTRENE, and ASTROSORBs 8, 25 andM3 sheeting [available from Astron Elastomerprodukte GmbH, Austria].Such materials are available in nominal 6 mm layers, but may easily beassembled in a thicker layers by adhesive bonding as required. Theaddition of one or more layers of these materials to an aerogelmultilayer absorber can significantly increase the absorption withoutgreatly affecting the reflectivity (if added to the side away from thesound source), but does add significantly to the cost and weight.

The graded acoustic absorber of the invention may also have its acousticproperties further modified and enhanced by pre-filling it with a gaswith relevant properties differing from those of air, in the manneralready described hereinbefore. Depending on the effect required, thisgas can be, for example, neon, hydrogen, helium or sulphur hexafluoride.

Although aerogel sound absorbers generally, and more specifically thesmall particles and graded aerogel absorbers of the invention, havehereinbefore been described primarily in connection with theirutilisation as resistive terminations for use with transducers, it willbe clear to those versed in the Art that a low reflectivity absorberstructure as so described above has application beyond that function. Inparticular, not only may such an absorber be effectively employed toabsorb sound from any source, but in addition, and unlike most absorberspresently in use, it has the capability of operating effectively even atlow audible frequencies, well below 400 Hz and extending all the waydown to 20 Hz. Such additional applications include architecturalacoustic control of rooms and buildings, reduction of sound emissionsfrom machinery and plant, soundproofing of rooms, buildings andvehicles, ear defenders, and indeed almost any application where theabsorption (rather than simply low transmission) of sound energy isimportant. In all of these, as well as the primary applicationenvisaged, the very low density of aerogel materials contributes to theoverall light weight of the absorber structure which is highlyadvantageous in some applications, especially where moving or movableparts are involved, e.g. vehicles and portable loudspeakers/earphones.

AEROGELS AS TRANSDUCER DRIVE ELEMENTS

One aspect of this invention relates to the use of aerogels astransducer elements.

There has above been described the use of aerogels as resistiveterminations in acoustic transducers. It is possible, however, to use anaerogel as the material from which is made a more active part of atransducer, namely the drive element itself.

As observed herreinbefore, an acoustic transducer of the “loudspeaker”form is basically an electromechanical device in which a diaphragm isdriven by a “translator”—a moving coil or a magnet itself driven by anapplied magnetic field. Magnets tend to be rather heavy, so perhaps thebest loudspeakers are most commonly of the light weight moving-coildesign, and can be made to operate up to and beyond the limit of humanhearing (20,000 Hz or more); the lighter the coil the more easily it canbe moved, and thus the higher the frequencies that can faithfully bereproduced by the loudspeaker.

Now, aerogels are extremely light, and if they were made to have asuitable magnetic character they could be used as the moving magnet in amoving magnet transducer to operate at either higher frequencies orhigher efficiencies than has hitherto been possible.

Such aerogels have now become possible, and therefore, in yet anotheraspect of the invention there is provided a moving magnet transducerwherein the moving magnet element is a magnetic aerogel structure.

More particularly, the invention here provides a piston or translator,forming the moving part of a linear electromagnetic transducer, beingmade of a magnetic form of aerogel—either magnetically-dopednon-magnetic-aerogel or intrinsically magnetic-aerogel—and beingsuspended in a controllable magnetic field as for example produced byone or more current carrying coils, so as to form a linear motor withvery low moving mass.

The common aerogels are usually made of silicate material, and have beenreported with densities as low as 3 kg/m3 (thus, just three times denserthan air). Typical examples are Matsushita's SP-15, SP-30 and SP-50mentioned above. Magnetic aerogels have been demonstrated bydistributing particles of ferrite or other magnetic materials throughoutthe aerogel structure; typical such magnetic aerogels are those producedby the Microstructured Materials Group of Lawrence Berkley Laboratories(USA) by chemical deposition of iron oxide (Fe₃O₄) into silica aerogel,and while this necessarily results in an increased-density aerogelstructure nonetheless relatively very-low density magnetic translatorsfor moving-magnet transducers can be made from such magnetic aerogels,and have shown promise in the manufacture of higher-frequencytransducers.

However, in the present invention the preferred magnetic aerogels aremade directly from materials—other than silicate—that are inherentlymagnetic. Such materials are Fe₃O₄, neodymium/iron/boron compounds,cobalt/samarium compounds, and other materials known to have usefulmagnetic properties, including many metal oxides, and their use can giverise to extremely low density magnetic materials from which may befabricated the desired transducer elements, translators for movingmagnet transducers.

Whilst the magnetic aerogel material, and the element made therefrom, isrelatively weak structurally, it should be noted that with thisconstruction the driving force acts throughout the body of the material(it all being magnetic), and thus there are no internal driving forcestending to distort the translator element. As a result, the element maybe made of extremely light construction, for great stiffness isunnecessary.

In this aspect of the invention the “magnetic” aerogel is fashioned,preferentially as a monolithic aerogel part, into a suitably shapedtranslator which, when driven by the magnetic field of an adjacentcurrent-carrying electrical conductor (conveniently formed into a coilof many turns), experiences a motive electromagnetic force, and ifsuitably suspended (by, for example, a bearing of some kind) then movesunder the influence of that force. The magnetic aerogel element, in thetransducer of which it is a part, may move the surrounding air eitherdirectly or via an attached diaphragm structure chosen to increase itcross sectional area in the plane orthogonal to the direction of motion.

One suitable form of suspension is a linear bearing of the type thesubject of Hooley British Patent Application No. 2,322,232 (P1481Sub),which adds almost no moving mass, seals the gap between the moving andfixed components, is silent in operation, and is almost frictionless.Another useful form of suspension for this device is to use a pair ofinverted thin-walled elastomer tubes attached one towards each end ofthe translator on the inside, and to the outside supporting framework onthe outside. Again, such a suspension can provide very long travel withlittle reactance, low mass, and low noise, and can be designed to sealthe gap between the translator and its surrounding framework. Theseparticular bearings have the added advantage in this assembly of actingover a large area of the body of the translator, and thus spreading theforces imposed on the translator body, which again is therefore notrequired to have great strength, allowing the use of inherently lowstrength aerogel materials.

For the aerogel translator described above to work most effectively asan acoustic ‘piston’ it is desirable that it should be made largelyimpervious to the surrounding air. This may be achieved by coating theaerogel with a thin, light, skin of gas-impervious material, such asvery thin plastic film, or very thin metallisation. A variant on this isto enclose the aerogel translator in a thin impervious bag or sack (orcovering) and then evacuate some or all of the air from inside thecovering (and necessarily also, from within the body of the aerogel),after which the covering is sealed. Where the aerogel had a very lowdensity (perhaps as low as 3 kg/m³), removing the enclosed air wouldfurther significantly reduce the mass of the aerogel translator as awhole, the static pressure of the atmosphere on the covering and thus onthe aerogel itself providing additional beneficial pre-stiffening of thetranslator component. A suitably thin impervious coating, of plastic forexample, can be made so as to add negligible additional weight to thetranslator. Typical materials widely available for this coating, or forthe bag or sack, or PVDC (polyvinylidene chloride), PVDF (polyvinylidenefluoride), PET (polyethylene terephthalate) polymers, possibly with anadditional very this metallised coating to improve their gasimpenetrability.

An additional useful adaptation of this idea is to mount the magneticaerogel on rotary bearings, and to arrange suitably-placedcurrent-carrying coils around it to form a rotary electric motor. With avery light, and thus very low inertia, rotor, such a motor has very gooddynamics. In this case, then, the aerogel transducer element is beingused in a “rotary” transducer, converting electrical energy not tolinear motion (and, by driving air before it, to sound) but instead torotary motion.

In this way the invention provides a rotor for an electric motor, whichrotor is constructed of a magnetic form of aerogel—one which is eithermagnetically-doped non-magnetic-aerogel or intrinsicallymagnetic-aerogel—and is in use suspended in a controllable magneticfield as for example produced by one or more current carrying coils, soas to form a rotary motor with very low moving mass.

AEROGELS AS TRANSDUCER ELEMENTS IN VISIBLE-IMAGE-FORMING DEVICES

In another aspect still the invention is concerned with aerogels usefulas parts of imaging systems.

Much effort is presently being expended to provide a commercially andtechnically acceptable form of three-dimensional television. One avenueof approach is by way of a projection hologram, and some experimentalsystems have been disclosed, while another is to employ a solid, 3D“image tank”—rather like a 3D version of a conventional televisionscreen—within which the image is constructed to have not only height andbreadth but also depth. The technique usually proposed is a tank, orsolid block, of a transparent material that can be caused to fluoresce(or glow by phosphorescence, like a conventional TV screen) whenimpacted or illuminated by two or more energy beams—focused electronbeams, say, or laser light beams—but not when impacted by only one suchbeam. Here, too, there has been some success—a block of the rightmaterial can be caused to glow where two laser beams meet to define apoint within the block, but not merely where a single beam traverses theblock but there are serious problems with finding the right blockmaterial. These problems are mostly centred on the fact that all knownusable materials—materials that will glow as required—are relativelydense, and a block that is of a decent size, giving acomfortably-viewable image, at least a foot (30 cm) across and deep, isextremely heavy.

It is here that the invention plays its part, by suggesting that therecould be utilised as the block material a monolithic transparent aerogelof a sort capable of fluorescence with the right stimulation. By probingsuch a block with one highly-focused, or alternatively with two or moreintersecting, possibly pulsed, possibly invisible (e.g. infra-red orultra-violet) laser beams arranged such that at their focus or point ofintersection they produce the appropriate conditions to stimulatefluorescence in the aerogel, a point source of light may be createdanywhere within the aerogel monolith and visible through four pisteradians. By rapidly scanning the laser beam or beams a 3D image canbe projected within the areogel monolith, and with an adequate updaterate moving images can be so produced. The great advantage of thisscheme over others using 3D point addressing in a transparent materialis the very low density of the aerogel, which thus allows very largedisplay sizes (e.g. one meter cubed) but with practical weights (lessthan 100 Kg and perhaps as low as 10 Kg—a mere 22 lbs, less than theweight of many present-day television sets). In addition, compared toany similar scheme using a gas as the fluorescing medium, significantlyimproved light output would be expected from an aerogel system asdescribed.

In this aspect, then, the invention provides a form of three-dimensionalvideo display wherein there is a solid, 3D “image block” within whichthe image is constructed by scanning the block internally with one ormore suitable stimulating energy beam, the block being formed of atransparent material that can be caused to glow when suitably impactedor illuminated the beam(s), and wherein the block is an aerogel.

In this aspect the invention provides an image-forming device which isessentially a monolith of transparent, visibly fluorescent aerogel. Itis drivable by one tightly focused, or two or more intersecting,possibly invisible (e.g. infra red or ultra violet) energy (convenientlylaser) beams, and is so arranged that, at the point of beam focus orintersection, the aerogel there fluoresces.

By suitable choice of either or both of the energy beams sources and thenature of the fluoresecent material within the aerogel, the imagingarrangement as described above can be given the additional property thatthe colour of the light emitted by the fluorescence can be varied eithercontinuously over the visible range, or alternatively so as to includeat least the colours red, green and blue, when stimulated by eitherdifferent wavelengths or by different intensities, or different meanenergies, or different combinations thereof, of a laser beam or beams,so as to form a ‘full-colour’ three-dimensional addressable display.

Several possible shapes are useful for such a display. A rectangularblock may be used for viewing principally from one of a number of fixedpositions, eg adjacent to the centre line of each of the six faces. If,however, it is desired to view the display by moving around it, then aspherical, spheroidal, or ellipsoidal block of aerogel has distinctadvantages as it lacks the corners of a rectangular block, and thusavoids the distortion in viewing produced by moving around a rectangularblock.

The aerogel itself can be any suitable transparent aerogel. One such isMatsushita's SP-50, and others are noted in the Aerogels 5, Proceedingsof the Fifth International Symposium on Aerogels, Publication mentionedhereinbefore.

PIEZOELECTRIC DEVICES AS ELECTROMECHANICAL DRIVERS

Another aspect of the present invention relates to novel constructionsof piezoelectric devices for use as electromechanical drivers.

Transducers which convert electrical energy to mechanical energy arewell known and come in a wide variety of forms perhaps the most commonof which is the loudspeaker (which converts electrical signals into themotion of a piston or like device the movement of which is caused todisplace air so as ultimately to “change” the electrical signal intoaudible sound).

Linear actuators of many varieties are also well known, examplesincluding hydraulic, pneumatic and internal-combustion cylinders,electromagnetic solenoids, linear motors of many kinds, piezoelectricand magnetostrictive actuators, and inch-worm devices.

Where a relatively small, self-contained, compact andelectrically-operated linear actuator is required capable of movement ofmerely a few millimeters and/or of the application of small forces inthe Newton range, then solenoids are generally preferred. It would beattractive to utilise a piezoelectric device instead, but unfortunatelythe present-day piezoelectric devices generally have a problem producingmillimeter-range displacements if direct acting, and even ‘stacks’ ofpiezoelectric plates produce only small deflections with practicalapplied voltages. Moreover, piezo devices become quite bulky if used inthe ‘bender’ mode; in this mode it is usual to provide an elongatetwo-layer cantilever beam made of a piezoelectric unimorph (a singleshape-changing layer on a shape-fixed layer) or bimorph (twoshape-changing layers back to back), which beam bends significantly asthe activating voltage is applied, but such a beam necessarily extendssome distance away from the axis of output movement.

Nevertheless, some use of piezoelectric benders in place of solenoidshas been made, particularly in the application of pneumatic valves,where a multilayer bimorph has been used to provide reasonabledeflection from relatively low operating voltages. Moreover, in theSpecification of the Hooley British Patent Application (P1481Sub)aforementioned there is described a helical tape-wound bender geometrysuited for applying radial “squeezing” pressures to a translator mountedtherewithin via a linear bearing. This helical bender has electrodes onits radially inner and outer surfaces which are split half way along theaxis of the helix and are cross-coupled to produce, when driven, areduction in helix radius at one end and an increase in helix radius atthe opposite end. The present invention concerns a more generally usefulmanner of substituting a piezoelectric device for a solenoid—ofretaining the compact cylindrical shape of the solenoid with the mainlength of the actuator aligned along (rather than at right angles as ina classical bender) to the direction of movement.

The helically-wound piezo device described in the aforementionedSpecification is one wherein the piezoelectric material is tape-wound,as though a length of tape or ribbon had been wound in the conventionalfashion around the outside of a cylinder. By contrast, in this aspect ofthe present invention a piezoelectric bender is made in the form of aflat- (or edge-) wound helical coil, with the direction of the appliedelectric field being between the two surfaces of the flat-winding (i.e.nominally aligned along the direction of the axis of the helical coil).

To best visualise the geometry of this structure, consider first aconventional elongate rectangular unimorph or bimorph piezoelectric‘bender’, with a thickness t (smallest dimension), width w (intermediatedimension) and length l (greatest dimension). The morph is constructedof two layers (which together have the total thickness t), and theapplied electric field is in the direction of the thickness, so that ifV volts are applied then the electric field in the morph has magnitudeV/t. Such a field will cause the morph to bend preferentially in adirection at right angles to the length and width dimensions, such thatthe thickness direction lies within that plane of bending. Next,consider the undeflected rectangular morph lying flat on a horizontalsurface, with the thickness t vertically aligned, and placed with itsside at one end adjacent to and touching a cylinder of diameter dstanding on the surface with its axis also vertical. Now imagine thatthe morph is flexible, and with the end touching the cylinder heldstationary, the morph is edge-wound around the cylinder (i.e. in theplane of the surface) so that it forms after one turn an annulus—acircle of inner diameter d and outer diameter approximately d+2 w. If,as the morph is wound around the cylinder, it is raised by at least aheight t per turn then it is possible to continue winding it into acontinuous helix with each turn being on top of its preceding turn, thusgiving a helix of pitch p (where in this case p=t). If the pitch p ismade greater than t then there will be a space between each turn ofwidth p−t. That describes the geometry of the helical bender of theinvention; the thickness (or smallest dimension) is alignedsubstantially axially along the helix, the width (or intermediatedimension) is aligned radially along the helix, and the length (orgreatest dimension) is aligned helically along the helix, and inoperation the morph is polarised in the thickness direction.

Note, incidentally, that it is not being suggested that the above methodis necessarily a practical means of constructing such a helical morph:this is merely a description to portray the desired geometry (however,it is in practice possible to make a helical morph in much this mannerif the winding into a helix is done while the ceramic layers of themorph are still in the green or unsintered state, and in practice if thegreen ceramic is suitably plasticised).

In this aspect, therefore, the invention provides a unimorph or bimorphpiezoelectric ‘bender’ formed into a flat- (or edge-) wound helix.

More particularly, this aspect of the invention provides a unimorph orbimorph (morph) piezoelectric ‘bender’ (bender) formed into a‘flat-wound’ helix—i.e. where the thickness or smallest dimension of thebender cross section is aligned axially along the helix, the width orintermediate dimension of the bender is aligned radially along thehelix, and the length or greatest dimension of the bender is alignedhelically along the helix—where the pitch of the helix is greater thanthe thickness of the bender, and where the morph is polarised in thethickness direction (i.e. nominally along the axis of the helix), andwherein electrically-conductive electrodes are deposited along thelength of the bender on both of the largest surface area sides of thebender (i.e. on either side of the thickness direction) and are drivableby an electrical signal so as to cause the helical bender to exhibit adimensional change in the axial direction when so driven.

In use the morph is polarised in the thickness direction (i.e. nominallyalong the axis of the helix—a direction approximately parallel to theaxis of the helix). Consider the effect of applying an electric field inthis direction along the thickness dimension. Each part of the helicalbender will try to bend nominally in the thickness direction, and thisattempt will, because of the helical geometry, cause the entire helix tolengthen or shorten (if the pitch p is not greater than the thickness tthen the helix will not be able easily to shorten). There will also be asmall deflection orthogonal to the thickness direction which will mostlycontribute to a slight increase or decrease in the diameter of thehelical coil, but his effect will be small compared to the length changein the helix, due in part to the structure and alignment of the morphrelative to the helix. Depending on the chosen materials, the achievabledeflection along the direction of helix axis, per size of helical coil,is considerable compared to that achievable using a comparably compactbender beam or stack. For example, a practical morph of width 8 mm andthickness 1 mm formed into a helix of inner diameter 16 mm and outerdiameter 32 mm, with 12 turns at 2 mm pitch, will have overallcylindrical dimensions of 32 mm diameter and 24 mm length, but will havethe equivalent bender-length of a cantilever beam bender (of the samewidth and thickness) of approximately 910 mm (nearly a meter), and willdeflect well over 10 mm with practical drive voltages. This is quitecomparable to the performance possible with a similarly-sized solenoid(though perhaps with somewhat less output operating force), and such ahelical bender has the great advantage over a solenoid that onceactuated it requires no static holding current, and therefore dissipatesessentially zero energy as heat, whilst still producing a static outputforce. The bender also has very small inductance, and is free from theinductive switching transients that are a problem with solenoids.

In operation the morph is, as just described, polarised in the thicknessdirection, and it is convenient to effect this utilisingelectrically-conductive electrodes deposited along the length of thebender on both of the largest surface area sides of the bender (i.e. oneither side of the thickness direction), and then driving these by asuitable electrical signal so as to cause the helical bender to exhibitthe required associated dimensional change in the axial direction.

One method of constructing such a bender involves co-extrusion of two(or more) layers of plasticized piezoelectric material, typically a leadzircondium titanate (PbZTi, or PZT) ceramic, to form a unimorph, bimorphor multimorph. The extrusion is effected through a rectangular aperturenozzle of exit dimensions w x t and so arranged internally thatextrudate issues from the exit aperture at a rate which is a function ofposition across the “w” dimension of the aperture; the effect of this isthat upon so exiting the extrudate “curls”, and in fact forms into acircle or helix if so coerced by means of an external cylindrical formerof diameter “d” and winding arrangement, with inner diameter d and outerdiameter d+2 w. In order successfully to achieve such co-extrusion it isnecessary to grind very finely and uniformly the PZT powder (obtained,for example, from Morgan-Matroc), and to mix it thoroughly with asuitable plasticiser (eg polyvinyl acetate, PVA) and water.

The separate layers are loaded with more or less silver oxide to make aunimorph—active layers with a small proportion of silver oxide, say 2%,and conductive inactive layers with somewhat more silver oxide, say 20%.For a two-layer device, one active and one inactive layer areco-extruded together such that their total thickness is ≅t, and each hasa thickness of ≅t/2 after extrusion. For a multilayer device, activelayers are alternated with inactive layers, each of nominal thicknesst/n, to form a total bender thickness of t after extrusion.

Once the two- or multilayer extrudate has been extruded and “wound” ontothe former of diameter d, it is sintered on the former in a furnace at atemperature in the region of 900-1,000° C. Surface electrodes can thenbe added to any external active layers by, for example, the sputteringof a conductive material such as silver or aluminum.

PIEZOELECTRIC DRIVER DEVICES WITH INTEGRAL POSITIONING AND CONTROLMECHANISMS

This aspect of the invention relates to the field of actuators andsensors, and in particular to unimorphs, bimorphs and multimorphs madeof piezo-electric material.

Piezo-electric actuators of many types are well known in the art. Thedirect piezoelectric effect is generally rather a small effect, of theorder of 10⁻¹⁰ m/v, so that to get substantial deflections using theeffect either very high drive voltages are required or a large stack oflow-deflection piezoelectric devices must be used. In either case, theachievable deflections are limited practically to the low-micron range.

When a greater deflection is required, various configurations of“bender” are used. A bender is a two-layer device wherein apiezoelectric layer is laminated (and intimately bonded together) witheither a non-piezoelectric layer (making a unimorph) or a piezo-electriclayer (a bimorph). In a unimorph, when the piezoelectric layer expandsor contracts under the influence of a drive voltage, the laminate as awhole is caused to bend due to the differential deflection between thelaminate's layers. In the bimorph—two piezoelectric layers laminatedtogether—the two layers are poled and then connected to the drivevoltage in such a way that when one layer of the laminate expands due tothe drive voltage, the other contracts, and vice versa. In this way, thelaminate again bends due to the differential deflection between thelaminate layers.

It is also known to stack multiple layers such as have been described,either to achieve greater deflection output-force, or, by using multiplethinner layers, to achieve a comparable output force from a lower drivevoltage, or for both of these reasons.

In this way, such piezoelectric benders as are presently available canproduce substantial deflections—on the order of millimeters—with drivevoltage as low as 30V to 60V.

Unfortunately, piezoelectric materials able to produce large deflectionsper volt (“high activity” materials), are also prone to considerablehysteresis. The generally undesirable result of this is that an actuatorcomprising such a piezoelectric bender is difficult to controlprecisely, because the hysteresis eliminates the possibility of thereexisting a simple relationship between input drive voltage and outputdeflection.

A similar effect results from the fact that piezoelectric materials havenon-negligible compliance. In the case of their use as a bender-typeactuator, the external load applied to the actuator will also determineto an extend, the amount of the output deflection.

Thus, conventional benders have shortcomings for precision actuatorservice.

The present invention proposes a surprisingly simple solution to theseproblems. More specifically, it suggests in essence the addition to thebender structure of a further laminate layer of a piezo-activematerial—as explained below, this piezo-active material may bepiezo-electric or it may be piezo-resistive—the function of which addedlayer is (solely) for sensing and responding to the actual deflection ofthe bender device. If this additional sensing layer is made of a verylow hysteresis piezo-active material, then when the bender is deflected,the resulting activity response of the sensing layer will be largelyfree of hysteresis effects (the selection of the material for thissensing layer may be made on grounds of low hysteresis and highpiezo-active response alone, as it plays no active part in causing thedeflection of the bender). Furthermore, if this sensing layer be made ofas high-compliance a material as possible—either by choice of thepiezo-active material, or simply by making the layer sufficiently thin,or both—then the presence of the sensing layer minimally loads theprimary bender device, and so it has little effect on the device'soutput deflection. Finally, if the control system driving the bender beprovided with the appropriate inputs and outputs, it can then use theoutput signal from the sensing layer in a feedback loop to controlclosely the actual output deflection, largely free of hysteresis errors,and also independently of the mechanical loading of the device.

In this aspect, then, the present invention provides a bender structurepiezoelectric device wherein there is an additional laminate layer of apiezo-active material the function of which is for sensing andresponding to the actual deflection of the bender device.

Such a sensing layer may be added to any of the unimorph, bimorph,single- or multiple-layer benders of all kinds that are currently known,in order to provide direct feedback of the deflection state of thebender, and thus greatly to simplify the precise control of such abender. And the bender may be a flat- (or edge-) wound helical device asjust described, or it may be one of the tape-wound helical devices thesubject of the aforementioned British Patent Application.

The added sensing layer used in the present invention is a piezo-activematerial; this piezo-active material may be piezo-electric or it may bepiezo-resistive. A piezoelectric material is one that outputs a voltagewhen it it mechanically deformed (and, or course, vice versa: when avoltage is applied across it then it mechanically deforms). By contrast,a piezoresistive material is one the electrical resistance of whichchanges as it is mechanically deformed. In the present invention eithertype may be employed—with a piezoelectric material the associatedcontrol system detects the output voltage, while with a piezoresistivematerial the control system applies a voltage and detects the change incurrent and thus resistance, or applies a current and detects the changein voltage.

The sensing layer may be added to either of the outside layers of thelaminate structure, or indeed, added as one of the inner layers of thestructure. For example, it may be arranged to be the layer closest tothe neutral axis of the bender, generally near the centre of the benderlayer-stack, so as the better to detect the average deflection of thebender structure. Positioning the sending layer close to the neutralaxis also subjects the sensing layer to the least strain, and thusminimises hysteresis effects.

Although the bender structure of the invention need only have a singleadded layer, it can have two or more. Thus, two (or more) sensing layersmay be added to the bender structure in such a manner as to minimisehysteresis effects in the sensing layers by arranging for somecancellation of the hysteresis effects between the layers when used incombination. In one such arrangement, a differential amplifier can beused to sense the difference of the output signals from two such sensinglayers to provide a net signal from the sensing layers that moreprecisely indicates the actual deflection of the bender. When the addedlayers are piezoresistive sensing layers, they may each be arranged tosense the strain in the bender, whereafter, by suitable electronictechniques, their deflection sensitive signals may be made to add (forexample in a differential amplifier) while their temperature dependentand other non-strain-related resistance changes may be made to largelycancel in the electronics sensing circuitry.

Where benders are fabricated by bonding previously separate layers ofpiezoelectric material together, then the sensing layer may also bebonded into the laminate so constructed, and may, for example be made ofa totally different type of material optimized solely for its sensingrather than its deflecting properties. For example, both piezo-electricand piezo-resistive polymers have much higher compliance than equivalentthickness piezo-active ceramics, and whilst in some cases they are lesssensitive than some piezo-active ceramics they are nonetheless goodcandidates for this sensing layer application because of their low cost,high compliance, and the ease with which they may be bonded andfabricated.

Where benders are fabricated from two or more layers of piezoelectricceramic (bimorphs or multimorphs) and/or one or more layers ofnon-piezoelectric material together with one or more layers ofpiezo-active ceramic (unimorphs or multimorphs), and where suchlamination is carried out prior to the firing or sintering process (i.e.at the “green” ceramic stage), then it is still possible to add one ormore sensing layers of piezo-active ceramic at this same “green” stage,before firing, and to choose the material of the sensing layer on thegrounds of suitability for sensing rather than of deflecting under drivevoltage (e.g. the choice is for low hysteresis, and/or high compliance,and/or high sensitivity). Alternatively, the sensing layers may bebonded onto such “fired laminates”, after firing, in which casenon-ceramic sensing layers such as piezo-active polymers may be used inaddition to high-temperature-resistant piezo-active ceramics.

In particular, the piezo-active ceramic extrusion and calenderingprocesses pioneered by Pearce et al may be used to produce, in onemanufacturing process, a multilayer bender device complete withdeflecting layer(s) and piezo-active sensing layer(s), with electrodesfired in during one and the same process.

In the manner described, as piezoelectric actuator may be constructedsuch that, with a suitable control system able to make use of thedeflection feedback signal available from the sensing layer(s), it iscapable of precise control without the need for any additional externalposition sensing devices—and thus it can be made at significantlyreduced cost.

One preferred candidates for the piezoelectric sensing layer is the same(active) PZT material from which the underlying bender is fabricated, egMorgan-Matroc PZT-4D, PZT-5A or PZT-5H (=Navy Types I, II and VI).Alternatively, where the piezoelectric sensing layer is laminated bybonding onto the bender after the bender has been sintered, the sensinglayer may be made of PVDF piezoelectric polymer.

Lastly, if the bender itself is made of piezoelectric polymer (nosintering step) then the sensing layer—of, say, PVDF—may be added at thesame time as the bender is fabricated.

One preferred candidate material for the piezo-resistive sensing layeris ruthenium oxide. This material may be applied in some appropriatemanner as a thick film to a surface of a bender structure, possibly overthe top of an insulating thick-film layer, then “cured” (sintered) toform the required ceramic structure. Thick film layers of this kind havethe advantage of requiring only a relatively small quantity of thepiezo-active material, which permits the construction of a low-costsensing layer. Suitable application processes include the conventionalmethods of thick-film technology, including screen printing, doctorblading, painting and spraying techniques. In this case the piezoactivematerial is applied to the bender structure in the form of an “ink”being a mixture of, amongst other components, piezoactive, conductiveand binding agents. Whilst the piezoactive layer(s) may be embeddedwithin the bender structure, in a preferred embodiment the layer(s) areapplied post-sintering, to maximize signal and minimise reactions.

One particular application for a bender-type piezoelectric device of theinvention—one having an integral sensing layer—is an acoustictransducer, wherein the bender is arranged to produce movement in theair, and thus sound waves, in response to an input drive voltage (anelectrical signal representing the sound), in the manner of aloudspeaker. In general, because of the aforementioned hysteresis andload-dependent deflection effects, such transducers can be significantlynon-linear, which is generally undesirable, as it produces distortion inthe output sound. The providian of one or more integral piezo-activesensing layer easily and cheaply provides sensing signals which may beused in a feedback control system to eliminate the greater part of suchnon-linearties, and thus to minimise such acoustic distortions.

Piezoelectric transducers that can benefit from the present inventioninclude the type of device the subject of the Hooley Britian PatentApplication No. 2,322,232 (P1481Sub) aforementioned. This Hooley deviceis an alternative helical actuator which is more similar to a simplebender (but in this case a bender that has been coiled helically) thanto the Pearce device. If, however, there is added to the Hooley helicalbender actuator one or more additional piezo-active sensing layer—inthis case as an additional laminate layer coiled helically conformallywith the deflecting layers—the added layer(s) can then be used to sensethe deflection of the helical actuator and to provide a feedback signalable to assist accurate control of the actuator. In so doing, of course,there can be reduced the effects both of hysteresis in the piezoelectricmaterial of the actuator and also of the loading on the actuator. With asuitable feedback control system, substantially linear operation of theactuator is possible, making it suitable for use in, for example, anacoustic transducer.

Of course, if the piezoelectric material of a bender is operated largelyin the approximately linear region, and is so chosen to have as low ahysteresis as possible, then inherent hysteresis effects may be madequite small, at least for AC signals. However, for DC drives, and forpartially static loads, the addition of the sensing layer will provideinformation about the actuation point of the bender, by measuring thestrain, and can thereby provide signals able to be used to reducegreatly the effects of varying loads on the actuation point.

In order to achieve better linearity of the piezoactive sensing layeraround the zero strain point of the bender, the sensing layer may bebonded to the bender (in the post-sintereing method of construction)whilst the bender is maximally deflected in the direction that wouldnormally apply compressive strain to the piezoactive layer, and thebender deflection maintained until the bonding process is complete. Inthis way, during all regions of normal operation of the bender, thepiezoactive sensing layer will be in tension and so will for the mostpart operate well away from the zero-strain region.

PIEZOELECTRIC PRINT-HEADS

In yet another aspect the invention is concerned with a quite differenttype of piezoelectric actuator, namely that sort which is used inink-jet printing.

In this type of device there is formed a block of piezoelectric materialhaving a planar array of a large number of very fine, very closeparallel chambers therewithin into which ink can be fed at one end andfrom which ink can be pumped out of extremely small apertures at theother end. The chambers are provided with opposed electrodes on eitherside, and when these are activated the piezoelectric material deforms,reducing the volume of the relevant chambers, and so driving ink outthrough the apertures.

These piezoelectric print-heads are presently manufactures by alaborious, multistage process, as follows. First, a suitable thick sheetof piezo-ceramic has milled into one major face a series of elongateparallel slots, leaving between the slots thin walls upon which aredeposited electrodes. A top-plate of piezoceramic is then bonded on topof the milled face of the sheet, covering the slots and making bondedcontact with the tops of each of the slot-walls, thus forming by meansof this fabrication a series of parallel approximately rectangular tubeswithin the body of the composite structure. Finally, end plates arebonded onto the edges of the laminate perpendicular to the direction ofthe slots, to close off the tubes so formed, and into one theseend-plates are bored precision holes, often using a laser beam, to formextremely fine nozzles.

In use, ink is allowed to enter the chamber via additional holes intothe slotted chambers, and when a drop of ink is to be ejected theelectrodes on the slot side walls are driven electrically in such a wayas to deform the slot walls and reduce the volume of the associatedrectangular tubular chamber, this reduction in volume causing thevirtually incompressible ink to be ejected through the nozzle (someother arrangement is provided to prevent the ink being ejected throughthe filler holes). In practice the procedure is more complex than thisbrief description makes it seem; it involves, for example,carefully-timed waves being launched into the ink so as to cause therequired ink flow.

This type of structure is capable of producing very fine resolutionink-jet print heads, and is already highly developed. However, itscomposite multi-process construction makes it a high-cost item, and thenon-availability of large sheets of appropriate ceramic materialprevents efficient mass manufacture of multiple devices. The processinvolves: the grinding to flatness of the piezoceramic base plate andlid-plate (currently necessary to ensure good mechanical alignment andbonding); the milling operation to produce a series of slots in the baseplate; the metal deposition process to provide electrodes on the sidewalls of the cavities, and any ancillary intermediate cleaningoperations between milling and metal deposition; and the alignment andbonding of the top plate to the milled and slotted base plate and anyancillary intermediate cleaning operations between metal deposition andbonding.

It is further purpose of this invention to describe an alternative, andmuch simpler and cheaper, manufacturing method for such devices, and forother similar devices.

The invention proposes that an ink-jet print-head style device be madeby a simple multi-layer extrusion process of a type similar to thatpioneered by Pearce et al at the IRC for Materials, University ofBirmingham, to make co-extruded hollow PZT tubes and multilayer PZTbender structures complete with integral conductive electrode layers.Using an appropriately formed and dimensioned extrusion die togetherwith a multilayered composition of piezoelectric ceramic paste andplasticiser, some layers of which have been well mixed with aconduction-producing material (such as silver oxide) to make a highlyconductive but mechanically compatible material after firing, there isprovided an extrudate in the form of an arbitrarily long strip—itslength is determined only by the quantity of material to beextruded—having internally a multiplicity of tubular cavities, extendingthe length of the extrudate, the walls of which are the piezoelectricceramic paste/plasticiser/conductor composition which, when fired, formsa conductive electrode layer on the surface of those walls.

In this aspect, then, the invention provides a method for making achannelled piezoelectric device like that required for a piezoelectricink-jet print head, in which method a composition of piezoelectricceramic paste, plasticiser and a material which becomes electricallyconductive after sintering, which composition is capable of being firedto form a highly conductive but mechanically compatible material, isextruded through an appropriately formed and dimensioned extrusion dieto produce an extrudate in the form of an elongate strip havinginternally a multiplicity of tubular cavities extending the length ofthe extrudate, the walls of which cavities are the piezoelectric ceramicpaste/plasticiser/conductor composition which, when fired, forms aconductive electrode layer on the surface of those walls.

The invention in this aspect provides a method for making a channelledpiezoelectric device like that required for a piezoelectric ink-jetprint head. An instance of a device other than a print head is amicro-pump of the sort used either for metered drug administration orfor controlling sample flow in chromatographs.

The invention provides a piezo-device-manufacturing method in whichthere is employed a composition of piezoelectric ceramic paste,plasticiser and a material that makes the sintered composite ceramicelectrically-conductive. This composition is in use extruded, and isthen capable of being fired to form a highly conductive but mechanicallycompatible material.

Suitable piezoelectric material for making the ceramic include thoseMorgan-Matroc substances mentioned hereinbefore—Morgan-Matroc PZT-4D,PZT-5A or PZT-5H. They may conveniently be plasticised using PVA, andadding silver oxide to them makes them conductive after sintering.

The method of this aspect of the invention requires the composition tobe extruded through an appropriately formed and dimensioned extrusiondie to produce an extrudate. A co-extrusion die is most likely to beuseful for this purpose. Here, two material entry points are providedinto the die, which ultimately are extruded from the one and the sameaperture or set of apertures (in the case of a multi-nozzle ink-jet pumpdie). Material is forced into the input apertures—plasticized active PZTpaste into a first aperture, and a similar material but with theaddition of the well-mixed-in conductivity-providing agent (eg silveroxide) into the other. Within the die, the main body of the extrudate isformed from material from the first aperture; however, the die is soarranged that a thin layer of (potentially conductive) material from thesecond aperture is deposited to the side wall positions of each of theslots of rectangular holes that appear in the extrudate, thedie-internal pressures being such that the co-extruded materials, fromthe two input apertures are mutually in contact at the outputaperture(s) to form a single continuous but laminated body of extrudatethereat.

In this aspect's method the composition is extruded to produce anextrudate in the form of an elongate strip having internally amultiplicity of tubular cavities extending the length of the extrudate.Most commonly the tubular cavities will preferably be rectangular incross-section (the section is determined by the die apertures), butother convenient and easily-attainable section shapes are circular orelliptical (such sections are very difficult to produce by machining).

There is not practical limit to the length of continuous extrudatepossible with this process, as material may be fed continuously into theinput apertures. However, a practical limit of some tens of feet(several meters) for the sintering furnace dictates cutting theextrudate (extruded onto a carrier which provides support thereafteruntil completion of sintering) into corresponding lengths (or less). Ina direct replacement for the present manufacturing process (bymachining) the apertures may be rectangular.

The extrusion method produces an elongate strip having internally amultiplicity of cavities the walls of which are the piezoelectricceramic paste/plasticiser/conductor composition which, when fired, formsa conductive electrode layer on the surface of those walls.

The co-extruded post-sintered conductive layers will generally be madeas thinly as careful process control allows, but in any case in the10-250 micrometer range. The layers on either side of the active PZTmaterial forming the walls between the rectangular cavities, becomeconductive after sintering, and so provide the means by which in thedevice's use those walls can be electrically driven (and therebydeformed).

It will be seen that by means of the invention's simple, continuous,extrusion process there can be produced in one single operation, and ina manner that eliminates a great deal of complex and expensiveprocessing (and so is at a considerably reduced cost), the types ofstructures needed for the variety of ink-jet print head previouslydescribed.

Whereas in the presently-used methods the whole assembly is required tohave precise dimensional accuracy to allow registration of the separatecomponents, in the method of construction of the invention there is farless requirement for absolute precision, as ultimately the only partthat requires significant precision alignment are the ink-jet nozzles(which may be bored in an end plate in a similar manner as at present,and thus each can be positioned relative to the other nozzles withadequate precision by that process alone).

It will also be appreciated that the novel form of construction proposedhere may also be used as the basis for a wide range of fluid pumpingdevices other than ink-jet print heads, to which the disclosed techniqueis in no way limited.

Embodiments of the several aspects of the invention are now described,though by way of illustration only, with reference to the accompanyingdiagrammatic Drawings in which:

FIG. 1A shows a section through a loudspeaker utilising asound-absorbing aerogel plug of the invention;

FIG. 1B shows in section a graded aerogel suitable for use in theaerogel plug of FIG. 1A;

FIGS. 2A & B show longitudinal- and cross-sections of a transducerdevice utilising a magnetic aerogel translator acting as a piston;

FIGS. 3A & B show longitudinal- and cross-sections of an alternativeform of transducer device similar to that of FIG. 2;

FIG. 4 shows a perspective view of an aerogel-utilising imaging systemof the invention;

FIG. 5 shows a perspective view of a form of piezoelectric helicalbender of the invention;

FIGS. 6A & 6B show both a perspective view of an alternative version ofpiezoelectric helical bender, using a piezo-active sensor layer, andalso a section through a more conventional type of bender but using asensor layer according to the invention;

FIGS. 7A & 7B show both a perspective view of another alternativeversion of piezoelectric helical bender, using a piezo-active sensorlayer, and also a section through a more conventional type of bender butusing a sensor layer according to the invention;

FIG. 8 shows a circuit diagram of a simple feedback control system foruse with piezoelectric benders such as are shown in FIGS. 6 and 7;

FIG. 9 shows a section through a simple piezoelectric ink-jet print headdevice according to the invention.

FIG. 1A shows schematically an acoustic transducer (5: cone/diaphragm5A/5B supported within protective casing 5C) mounted and sealed into theend of a gas-impermeable tubular support (6), the transducer 5 emittingsound from its front face (9) and rear face (10). The translatorcomponent (11: 5A/5B) of the transducer 5 is assumed to begas-impermeable also.

Behind the tranducer 5, and sealed into the support 6, is a plug (7) ofaerogel whose density, material and pore size are chosen preferentiallyto absorb low frequency sound highly, and to absorb in particular thatsound emitted from the rear face 10 of the transducer 5. The front face(13) of the aerogel plug is open to the rear of the transducer 5, whilstthe rear face (12) of the aerogel plug is sealed from the surroundingatmosphere by a loose acoustically transparent gas-impermeable membrane(8) which is sealed at its periphery to the support 6.

The space enclosed by the rear of the transducer 5, the inside surfaceof the support 6 and the inside surface of membrane 8 is filled with agas (or mixture of gases) chosen to maximise the absorption of lowfrequency sound from the rear of transducer 5. The arrangement is suchthat the destructive interference at the front of the transducer by therear radiation is reduced by the aerogel absorber 7, whilst the acousticloading on the rear of the transducer is minimised.

FIG. 1B shows in section, and for use in an absorber system like thatshown in FIG. 1A, a section through an aerogel plug (17, much like theplug 7 in FIG. 1A). Here, the aerogel plug 17 is structured as a numberof layers (as 17A,B,C,D and so on: not all the possible layers areactually depicted in the Figure), each layer comprised of materialdiffering in one or more physical and/or chemical characteristics fromthe adjacent layers, chosen so as to reduce the reflection of incidentsound on face 13, whilst, subject to that first constraint, maximisingthe absorption of sound energy incident on face 13. In anotheralternative structure, not shown here, the individual layers may mergesmoothly with one another to become a continuously graded region ofmaterial with the principal direction of grading of material reflectioncoefficient increasing in the direction away from transducer 5.

FIGS. 2A,B show a cylindrical magnetic aerogel translator (32) withsealed surface, acting as a piston, freely suspended inside a rigid thintubular member (34) (seen here in section only) by an inflated toroidalpneumatic bearing (33) (seen here in section only), there being wound anelectrical coil (31) (seen here in section only) around the outside ofthe member 34. The translator 32 is magnetised in one of many possibleconfigurations—for example, along the direction of the axis of member34, and when current passes through the coil 31 the field of the coilinteracts with the field of the translator 32 to provide an axial forcewhich drives the translator one way or the other along the axis.

FIGS. 3A,B show an alternative configuration of device similar to thatshown in FIGS. 3A,B.

Instead of a toroidal bearing there is a pair of inverted thin-walledelastomer tube sections (300,301) which are unattached to anythingexcept at their ends: inverted tube 300 has its outside end attached tothe inside of tubular member 34 (at location 302) and its inside endattached to translator 32 (at location 304), while the other invertedtube 301) is similarly attached at the other end (at locations 303,305to the tubular member 34 and to the translator 32 respectively). Whenthe translator 32 moves axially relatively to the tubular member 34 theinverted tubes roll inside themselves, allowing nearly free movement ofthe translator.

The relation between the external diameter of the translator 32, theinternal diameter of the tubular member 34 and the natural (unstrained)inside and outside diameters of the two tubes 300,301 is critical, andshould be chosen such that in the assembly shown there is a smallclearance between the outside of translator 32 and the insides of thetubes 300,301 (except where the latter are attached to the translator32) and a small clearance between the inside of the tubular member 34and the outsides of the tubes (again, except where the latter areattached to the member).

FIG. 4 shows a block of transparent, fluorescent aerogel (41), aboutwhich are placed a pair of scanning lasers (42,43) emitting narrowparallel beams of light (46,47) which are deflectable by means ofcontrollable optical beam deflectors (44,45). The beams 46,47 pass into,and scan through, the aerogel block 41.

The beam intensities or energies are individually insufficient to causethe aerogel to fluoresce. However, where the beams intersect, as at thepoint (48) shown, their effect is sufficient to cause visiblefluorescence in the aerogel. The colour of the fluorescence iscontrollable by modification of the laser beam wavelength, and/orintensity, and our pulse duration if pulsed.

By appropriate deflection of the beams every position within the aerogelmonolith can be separately caused to fluoresce, and thus a 3D displaydevice is created.

A variant of this system, not shown here, uses a similar block oftransparent, fluorescent aerogel adjacent to which is placed a singlescanning laser the beam of which is highly convergent and controllablyfocusable, such that the point of focus, where the energy density ishighest, may be positioned anywhere within the aerogel block. The focus,beam and energy density are arranged such that only very close to thepoint of focus is the energy density adequate to cause substantialfluorescence in the aerogel. Thus, by controlling the position of thebeam and its focal point, a visual image can be built up in the aerogelblock.

FIG. 5 shows a helical flat-wound bender of some suitable diameter,thickness, pitch and width (shown respectively at 58, 57, 59 and 56).

The bender is comprised of a top (a viewed) layer (54) and a bottom (asviewed) layer (55) bonded together at their interface (50). If bothlayers 54,55 are piezoelectric then the bender is a bimorph; if only onelayer is piezoelectric then it is a unimorph.

The helix extends or contracts along the direction of the axis (shown asdashed line 51) depending on the polarity of the electrical drivevoltage applied between conductive electrodes (not shown) deposited oneon the top (as viewed) face of the top layer 54 and one on the bottom(as viewed) face of the bottom layer 55. To allow easy application ofload forces, the top and bottom turns of the helix may be flattened outsomewhat as indicated in the Figure, or they may be ground flat.

FIGS. 6A,6B show simple bimorph “benders”, one “helical” (like that ofFIG. 5) and one “linear”, to each of which has been added a sensinglayer.

FIG. 6B shows a simple bimorph bender of conventional type, wherein twolayers of piezoelectric material (61,62) are bonded together, with anoptical electrode (64) between them, and electrodes (65,66) attachedsuch that a drive voltage may be applied to the opposing faces of thestructure so formed. An additional layer (63) of piezoelectric materialis the sensor layer; it plays no part in the bender deflection processwhen layers 61,62 are driven (other than to impede it somewhat by virtueof its finite compliance). An additional electrode (67) is arranged onthat face of sensor layer 63 not bonded to layer 62, in order to providea signal output from this sensor layer when deflected by the benderaction of layers 61,62.

In the device as shown, the sensing layer 63 shares a common electrode(66) with the two main layers 61,62. In applications where thiselectrode sharing is undesirable, it is possible to replace the singleelectrode 66 with a pair of electrodes insulated from each other by athin intervening layer, one of which provides connection to the benderproper layer 62 and one of which provides connection only to the sensinglayer 63. However, it will often be adequate to use electrode 66 as acommon ground for driving and sensing connections with littleinterference then generated in the sensing circuit by the drivingcircuit, provided care is taken to ensure that electrode 66 isadequately conductive.

In FIG. 6A there is shown a helical bender much like that of FIG. 5,with two electrically-driven piezoelectric layers (609, 610: theelectrodes are not shown), but with the addition of a furtherpiezoelectric laminate layer (608). This additional layer plays noactive role in deflecting the structure, but is used instead to providea feedback signal to sensing electronics (not shown) about the actualdeflection of the bender when in use. The added “passive” sensor layer608 also has surface electrodes (not shown), one of which may be sharedwith the active layer 609 if desired, as described above for the simplebender case.

In use, a voltage is driven between the top (as viewed) of active layer609 and the bottom of active layer 610, which layers have beenpreviously poled in an opposite sense to each other. This causes theribbon-like structure (which has been edge-, or flat-, wound into ahelix) to bend, and this deflection causes the helix as a whole tolengthen or shorten defending on the sign of the drive voltage. Thesensing layer 608, being securely bonded to (or fired onto) active layer609 is also deflected by this bender activity, and in so being, andbecause it is itself piezoelectric, produces a voltage signal betweenits surfaces which may be used as a feed back signal connected via itssurface electrodes (not shown).

FIGS. 7A,B are in many respects very like FIGS. 6A,B.

The simple bimorph bender of FIG. 7B is of a generally conventional typewherein two layers of piezoelectric material (71,72) are bondedtogether, with an optional electrode (74) between them, and electrodes(75,76) attached such that a drive voltage may be applied to theopposing faces of the structure so formed. Also shown in FIG. 7B is anadditional layer of piezo-resistive material (73) which plays no part inthe bender deflection process when layers 71,72 are driven (other thanto impede it somewhat by virtue of its finite compliance), which layer73 is bonded to electrode (layer) 76 with an intervening insulatinglayer (not shown) to electrically isolate it therefrom. Additionalelectrodes (77,78) are arranged on that face of layer 73 not bonded tolayer 72 (via the insulating layer and electrode 76), one on each end ofthe piezo-resistive material of the layer 73, in order to provide asignal output from this layer when deflected by the bender action oflayers 71,72.

FIG. 7A shows a helical bender much like that of FIG. 5, and with twoelectrically-driven piezoelectric layers (709,710: the associatedelectrodes are not shown), save that it includes a further piezo-activelaminate layer (708: in this embodiment this further piezo-active layer708 is piezo-resistive, unlike the case of the FIG. 7A embodiment, wherethe sensing layer is piezo-electric). This extra layer plays no activerole in deflecting the structure, but is used instead to provide afeedback signal (to sensing electronics, not shown) about the actualdeflection of the bender when in use. The sensing layer 708 is bonded toactive layer 709 and its electrode via an intervening insulating layer(not shown) so as electrically to isolate the sensing layer 708 from theactive layer. The sensing layer 708 also has electrodes (again, notshown), one at each end of the helical structure, for connection to thesensing circuitry.

In use, a voltage is driven between the top (as viewed) of active layer709 and the bottom of active layer 710, which layers have beenpreviously poled in an opposite sense to each other. This causes theribbon-like structure (which has been edge-, or flat-, wound into ahelix) to bend, and this deflection causes the helix as a whole tolengthen or shorten depending on the sign of the drive voltage. Thepiezo-resistive sensing layer 708, being securely bonded to or firedonto active layer 709, is also deflected by this bender activity, and inso doing, and being piezo-resistive, produces a resistance change alongits length which may be converted to a voltage signal by the passage ofa current through it (via its electrodes, which signal may be used as afeed back signal to sense and/or control the deflection of the helicalbender.

FIG. 8 shows a typical driver circuit incorporating a “bender” withpiezo-resistive sensor layer feedback.

In the Figure there is shown a differential operational amplifier (816)is used in a classic negative-feedback circuit to drive a bender (815),from an input demand signal (fed in at input 818) which is connected tothe inverting input of the amplifier 816 via a resistor (817). In oneversion of this circuit, resistor 813 represents the piezoresistivesensing layer integrated with the bender 815, and a current is made topass through this resistor from a DC voltage supply (819) via a resistor(811). A voltage therefore appears across resistor 813 proportional toits resistance, and therefore with a component proportional to thestrain experienced by the sensing layer resistor 813. The voltage at thejunction of the resistors 811,813 is connected to the inverting input ofthe amplifier 816 as a negative-feedback signal. A further pair ofresistors (812,814) are used to produce an offset voltage the same valueas the voltage at the junction of the resistors 811,813 when the benderand sensing resistor are in the unstrained or undeflected state. Thisoffset voltage is connected to the non-inverting input of the amplifier816.

In operation, when the input voltage 818 is set to some demand valuewithin the range of operation of the circuit, the output of theamplifier 816 drives the bender to an operating point which causesstrain in the sensing layer piezoresistor 813 and changes its value.This modifies the voltage at the junction of resistors 811,813, which inturn modifies the differential input voltage applied to the amplifier816. If the sense of the resistance change is correctly chosen withrespect to the drive voltage applied to the bender 815 (i.e. thepolarity of the connection to the bender is such as to ensure thatnegative- and not positive-feedback is achieved), the circuit rapidlysettles to a point where the drive to the bender 815 is just such as toproduce a deflection or strain proportional to the input voltage 818.

An improved version of the arrangement shown in FIG. 8 may beconstructed as follows.

In this arrangement, both resistors 813,814 represent piezoresistivesensing layers integrated with the bender 815. They may, for example, beplaced one either side of the bender, but are in any case arranged suchthat one sensing layer experiences compression whilst the otherexperiences extension, and vice versa, of approximately equalmagnitudes. In such an arrangement, both piezoresistive sensing layerswill experience very similar magnitude strains (but of opposite signs)during operation of the bender, and both will be subject to similartemperature variations due both to environmental temperature changes andto changes in bender temperature caused for example by the drive powerapplied to it. In the circuit of FIG. 8 they are then connected suchthat their resistance changes provide feedback voltage of oppositepolarities to the two separate input terminals of the amplifier 816. Inthis way their sensing signals add together to provide increasednegative feedback. However, any changes in their resistance—due tocommon temperature variations, or due to long term ageing of thematerial—will tend to balance, and so produce approximately zerofeedback voltage. In this way a highly temperature compensatedintegrated actuator/sensor and control circuit may be achieved.

In a further slight variation on the last described circuit and deviceconfiguration, the resistors 813, 812 (and not 814) are bothpiezoresistive layers, and may be bonded on to the one and same side ofthe bender, each occupying roughly half the width of the bender and eachrunning the full length of the bender and thus experienced essentiallythe whole bender strain, but being electrically isolated from oneanother. In this configuration, strain in the bender 815 results insimilar magnitude and sign of piezoresistive changes in the two sensingresistors 813,812. Their location in the circuit of FIG. 8 results intheir strain-related resistance signals being additive, and causingnegative feedback.

FIG. 9 shows a section through a simple piezoelectric ink-jet print headdevice according to the invention (it is to be imagined that a longstrip of channelled body has been extruded, and has then been cuttransversely into usable lengths; the Figures shows one of the facesproduced by such a transverse cut).

The print-head has a piezoelectric body portion (911) within which are anumber of channels (as 912) defined by wall portions, and on thechannel-facing surfaces of these are conductive layers (as 913). Inoperation, ink is fed into each channel 911 via ports (not shown: theywould be in a blanking plate sealed over the cut face), and when a drivesignal is applied to the electrodes 913 lining any particular channel912 the walls flex, pumping some of the ink therein out through veryfine apertures (not shown: they would be in another blanking platesealing off the other end—also not shown—of the cut length).

What is claimed is:
 1. An aerogel, suitable for use as an absorber ofsound energy, which is constructed as a conglomerate of particles withthe packing of these particles being such that there are spaces lefttherebetween to allow the passage of sound.
 2. An aerogel as claimed inclaim 1, wherein the particles are from 0.5 mm to 20 mm across.
 3. Theaerogel as claimed in claim 1, wherein the particle shapes are selectedfrom the group consisting of spheres, cylinders, and irregular particlesizes as are produced by crushing particles or monoliths.
 4. An aerogelas claimed in claim 1, wherein the structure is stabilized by bondingthe particles together with an adhesive.
 5. A layered structure,suitable for use as an absorber of sound energy, comprised of multiplelayers of aerogels of graded values of acoustic reflection andabsorption properties, or comprised of aerogel material of continuouslyvarying values of acoustic reflection and absorption propertiesthroughout the thickness of the absorbing structure, in the principaldirection of sound from the source.
 6. A layered structure as claimed inclaim 5, which is successive layers of material overlaid onto asupportive substrate that is substantially acoustically transparent overthe frequency range of interest.
 7. A layered structure as claimed inclaim 5, wherein materials other than aerogels form one or moreadditional layers further away from the sound source.
 8. A layeredstructure as claimed in claim 5, which is pre-filled with a gas withacoustic properties differing from those of air.
 9. A layered structureas claimed in claim 5, which is an areogel suitable for use as anabsorber of sound energy, which is constructed as a conglomerate ofparticles with the packing of these particles being such that there arespaces left therebetween to allow the passage of sound.
 10. A layeredstructure as claimed in claim 5, wherein the aerogel is overlaid onto asupportive substrate.
 11. A layered structure as claimed in claim 5being pre-filled with a gas with acoustic properties differing fromthose of air.
 12. A layered structure as claimed in claim 5, wherein theaerogel particles are from 0.5 mm to 20 mm across.
 13. An absorber ofsound energy comprising at least one layer of aerogel, said absorberhaving a sound reflection coefficient at the input side interface whichsound reflection coefficient is smaller than 0.2 over the frequencyrange of interest.
 14. The absorber of claim 13, wherein the reflectioncoefficient is smaller than 0.2 in the 20 Hz to 20 KHz band.
 15. Theabsorber of claim 13, wherein the reflection coefficient is smaller than0.2 in the 20 Hz to 400 Hz band.
 16. The absorber of claim 13, whereinthe reflection coefficient is smaller than 0.2 at a sound frequency of30 Hz.
 17. The absorber of claim 13, having a layer of aerogel at theinput side interface, said layer being constructed as a conglomerate ofparticles with the packing of these particles being such that there arespaces left therebetween to allow the passage of sound.
 18. The absorberof claim 17, wherein the aerogel particles are from 0.5 mm to 20 mmacross.
 19. The absorber of claim 17, wherein the layer comprisesadhesive binder material.
 20. The absorber of claim 13 having a layer ofaerogel at the input side interface, said layer being constructed as aconglomerate of particles with the packing of these particles being suchthat there are spaces left therebetween to allow the passage of sound.21. The absorber of claim 13 having at least a second layer positionedfurther away from a sound source and having a higher sound absorptioncoefficient than the layer at the input side interface.
 22. The absorberof claim 21, wherein the at least second layer is formed of areogel. 23.The absorber of claim 21, wherein the at least second layer is formed ofnon-aerogel material.
 24. The absorber of claim 13 having a supportivesubstrate that is substantially acoustically transparent.
 25. A layeredstructure, suitable for the use as a sound absorber, comprising multiplelayers of areogel of graded or continuously varying values of acousticreflection and absorption properties throughout the thickness of theabsorbing structure in the principal direction of the sound from asource, wherein the layers of aerogel are constructed as a conglomerateof aerogel particles with the packing of these particles being such thatthere are left gas-filled spaces therebetween to allow the passage ofsound.
 26. The layered structure of claim 25, wherein the particles arefrom 0.5 mm to 20 mm across.
 27. The layered structure of claim 25,wherein the shapes of the particles are selected from a group consistingof spheres, cylinders, and irregular particle shapes as are produced bycrushing either larger particles or monolithic aerogel.
 28. The layeredstructure of claim 25, being stabilized by bonding the particle togetherwith an adhesive.
 29. The layered structure of claim 25, being overlaidonto a supportive substrate that is substantially acousticallytransparent over a frequency range of interest.
 30. The layeredstructure of claim 29, wherein the frequency range of interest is the 20Hz to 20 KHz band.
 31. The layered structure of claim 25 having at leastone layer of aerogel with a reflection coefficient smaller than 0.2 inthe 20 Hz to 20 KHz band.
 32. The layered structure of claim 25 havingat least one layer of aerogel with a reflection coefficient smaller than0.2 in the 20 Hz to 400 Hz band.
 33. The layered structure of claim 25having at least one layer of aerogel with a reflection coefficientsmaller than 0.2 at a sound frequency of 30 Hz.
 34. The layeredstructure of claim 25, wherein the gas-filled spaces are filled with agas with acoustic properties differing from those of air.
 35. Thelayered structure of claim 25, wherein materials other than aerogel formone or more additional layers further away from the sound source.